Fix missing browser.h includes

Currently, the MediaRouter BUILD.gn does not declare any dependencies on
the UI browser.h file, causing build failures on some bots. This patch
adds a dependency on //components/signin/public/base:signin_buildflags
to fix the Open Screen build bots, as ui/browser.h ultimately depends on
this generated header.

Change-Id: Iafed8a6a3717b9ee2b8df6fb41f385a4a0acc4bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1951543Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Commit-Queue: Jordan Bayles <jophba@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721667}

chrome/browser/media/router/BUILD.gn

@@ -71,6 +71,10 @@ static_library("router") {
   if (enable_extensions) {
     deps += [
       "discovery",
+      # We can't depend on //chrome/browser/ui due to introducing a cyclic dependency,
+      # so we have to depend on this directly to fix include resolution for
+      # browser.h, which is used in multiple extension-only files.
+      "//components/signin/public/base:signin_buildflags",
       "//components/mirroring/mojom:host",
       "//components/mirroring/mojom:service",
       "//components/translate/content/common",